Currently there is growing interest in investigating nanostructured semiconductors that function as CO2 reduction photocatalysts that utilize sunlight for generating fuels in an artificial photosynthetic device (e.g. Bensaid et al. ChemSusChem 2012, 5, pp 500-521 and Izumi Coord. Chem. Rev. 2013, 257, pp 171-186). Conversion of CO2 derived from fossil fuel-based energy and manufacturing waste streams into valuable products, such as carbon monoxide, methane, or methanol, would represent a huge economic and environmental benefit, simultaneously addressing issues of energy security and climate change. While artificial photosynthesis can exist in multiple configurations, gas phase photocatalysis has recently been identified in Olah et al. J. Am. Chem. Soc. 2011, 133, pp 12881-128980 as a scalable and economically feasible option for large-scale CO2 reduction. Artificial photosynthetic devices have been documented (Paul O'Connor U.S. Pat. No. 8,519,012 and Mengyan Shen, Cong Wang, Yeshaya Koblick, WO2013063064), however, each device is unique and functions under specific operating conditions. It is still unknown which materials compositions and properties are ideal to facilitate gas phase photocatalytic conversion of carbon dioxide.
A semiconductor photocatalyst is a type of catalyst that absorbs light in a manner which changes the surface chemistry of the semiconductor thereby providing a means to drive chemical reactions. Semiconductor photocatalysts are heterogeneous catalysts, which mean the reactant components exist in a different phase (liquid or gas) than the catalyst (solid). A functional photocatalyst must absorb light, preferably in the ultraviolet and visible spectral regions for solar powered applications. When a semiconductor photocatalyst absorbs light with energy greater than the electronic band gap of the semiconductor, excited electrons are promoted to the conduction band while the number of electron holes in the valence band is increased above equilibrium concentrations. These energetic charge carriers (photogenerated electron and electron hole (e/h) pairs in excess of equilibrium concentrations) can facilitate surface chemical reactions of interest. The photoexcited electron should have an electrochemical potential energy that is more negative than the reduction potential required to reduce carbon dioxide or a surface species originating from carbon dioxide. These e/h pairs must have a long enough lifetime to be able to diffuse to the surface of the semiconductor, with minimal recombination, in order to transfer or accept electrons from adsorbed molecules. Additionally, this material should have a favorable surface that preferentially absorbs reactants and desorbs products and must be stable under relevant reaction conditions.
Metal oxide semiconductors are a class of materials which satisfy the above conditions. These materials can be made of earth abundant elements and fabricated at industrial scales using existing technology. Notably, the physical dimensions of metal oxides can be easily controlled from the macroscale to the nanoscale, affecting material properties such as the electronic band gap, charge-transport, and surface area. Because of these properties, metal oxide nanomaterials have been used as photocatalysts; the most often reported and studied is titania, TiO2. Titania-based photocatalysts have been documented (Ekambaram Sambandan, Rajesh Mukherjee, Takuya Fukumura U.S.20130192976). Metal oxide semiconductors have been reported to use sunlight to decompose organic compounds and dyes in both the gas and aqueous phase (Linsebigler, et al. Chem. Rev. 1995, 735-758). They also have been used successfully in photoelectrochemical cells for water splitting. There is growing interest in designing a semiconductor photocatalyst that is capable of CO2 photoreduction (Navalón, Set al. ChemSusChem 2013, 6, 562-577), but much of the field is misguided since most studies do not perform isotope tracing experiments, for example using 13CO2, to verify the origin of the observed carbon-containing products (Yang, C.-C et al. J. Am. Chem. Soc. 2010, 132, 8398-8406). Because of ubiquitous carbon contamination from carbon-containing precursors, solvents and ligand additives used to control the nanostructure morphology, the validity of many of these results has been called into question. More recently a few studies have used isotope tracing experiments to validate their claims, most notably Yoshida et al. 13CO2 to validate the efficacy of their ZrO2 catalyst, activated with deep UV light, for CO production (Yoshida et al. Catalysis Surveys from Japan, 2000 4, 2,pp 107-114). Despite the growing interest and investment in the field, there are few examples of successful efficient gas-phase photocatalysts, particularly those active in the visible region of the solar spectrum, suggesting new approaches to materials discovery are necessary. One such approach that has been employed successfully is the intentional creation of oxygen deficient metal oxides via hydrogen treatment, which can generate active catalytic sites and mid-gap defect stares, enhancing both the visible absorption and photocatalytic activity of the material. The most notable example of this is black titiania, TiO2−xHx, which exhibits a substantial increase in absorption (83% of the solar spectrum) and activity for hydrogen generation (Chen, et al. Science 2011, 331, pp 746-750) clearly demonstrating the effectiveness of oxygen vacancies in enhancing photocatalytic activity. Another approach to increasing the photocatalytic activity of metal oxide nanomaterials is by improving the CO2 capture capacity of the nanoparticle surface. Several groups have demonstrated the efficacy of surface hydroxides at enhancing the affinity of CO2 for photocatalytic surface, with demonstrated enhancement of photocatalytic activity (Ahmed, et al. J. Catal. 2011, 279, pp 123-135).